Direct observation of conductive filaments from 3D views in memristive devices based on multilayered SiO2: Formation, Dissolution, and vaporization

Memristive devices, also known as memristors or ReRAMs, are promising candidates for accessing nextgeneration memory. In classic electrochemical metallization (ECM) theory, there are only two states of conductive filaments: formation and dissolution. In our experiment, we found that the metallic filaments also vaporized, leaving observable defects in a series of memristive devices based on a Cu-doped multilayered SiO2 electrolyte layer. Furthermore, the vapour from conductive filaments exfoliated adjacent single layers of multilayered SiO2. The morphologies of the conductive filaments in a memristive device (W/Cu-doped SiO2/Ag) were studied using an SEM (scanning electron microscope) instrument equipped with an FIB (focused ion beam) module. With the gradual removal of the electrolyte layer, cross-sectional images of the conductive filaments were captured from perspective, top and side views. Based on these images, a three-dimensional model of the conductive filaments was proposed. All the findings suggested that the SET and RESET processes were complex and involved the simultaneous formation, dissolution and vaporization of conductive filaments. The vaporization of the conductive filaments permanently changed the surface morphology of the devices. This model, presented at the end of our paper, explains the irregular phenomena that occurred in I-V measurements.